1. Field of the Invention
A wide range of air-compressing internal combustion engines that are used to drive utility vehicles exists. Different demands are made of the fuel injection systems of such internal combustion engines upon starting than when the engine is operating at its rated rpm. Yet in designing fuel injection systems, both demands must be met. Short triggering times of the control valves of an injection system are just as important as favorable production costs and a long service life of the injection system components, the latter being achieved by providing for a pressure equilibrium of the valve components.
2. Description of the Prior Art
Many variant embodiments of fuel injection systems are known. Examples that can be named are systems in which a piston with a restoring spring or some other pressure-generating component is provided. This component is preferably driven by a camshaft. In injection systems as a rule, a nozzle needle is provided, which moves between a lower or closing position and an upper position, and pressures are exerted in controlling fashion on the faces at the ends of the needle. As a rule, one or more control chambers are provided in such nozzle needles; furthermore, a nozzle needle is held in its lower position by a restoring spring.
Injection systems are also known that optionally include a fill diversion valve, which primarily controls the pressure in one of the control chambers of the nozzle needle, and also include a nozzle control valve, which primarily controls the pressure in the outlet of a further control chamber of the nozzle needle. Both of these valves can be embodied so that they can be switched either in coupled fashion or separately, and either electromagnetic, piezoelectric or magnetostrictive actuators can be employed. The valves can be actuated either directly or indirectly, and the valves can be both preceded upstream by throttle elements and followed downstream by throttle elements.
A fuel injection system which controls the pressure in the outlet region of a control chamber surrounding the nozzle needle is known from European Patent Disclosure EP 0 823 550 A1. The fundamental disadvantage of this arrangement will be described briefly now. At very low engine rpm, for instance upon starting of an internal combustion engine, the piston generates a pressure that is above the pressure level at which the sum of all the pressure forces on the nozzle needle just barely exceeds the force of the nozzle restoring spring. To make the pressure buildup possible, both valves are initially closed. At a certain time, however, the nozzle control valve is opened, causing the pressure in the corresponding control chamber to drop, and the sum of the forces on the nozzle needle bring about a motion of the nozzle needle in the direction of the upper position. By means of the nozzle that opens in the direction of the cylinder and by means of the opened nozzle control valve, a quantity of fuel now flows out that is greater than the quantity of fuel replenished at the piston. As a result, the pressure in the other control chamber of the nozzle needle drops, and this nozzle closes again, which is unwanted.
U.S. Pat. No. 5,819,704 discloses a remedy for the unwanted closure of the nozzle when the fuel volume flowing out is excessive. In this variant embodiment, the nozzle needle is equipped with a second seat. The second seat closes off the outflow from a first control chamber. In addition, by the suitable selection of throttle faces and pressure faces, it is attained that the pressure in the control chamber rises slowly, and beyond a certain pressure level, the nozzle needle lifts up just before reaching the upper position. This brief lifting up causes the pressure in the control chamber to drop immediately again, so that the injection is unimpaired. A disadvantage of this configuration of a nozzle needle with a second seat is, first, that a double-seat valve is more complicated and expensive to produce. Second, in this configuration the injection cannot be terminated at any arbitrary instant.
The advantages that can be attained with the embodiment according to the invention are considered to be above all that when a piezoelectric actuator is used, the briefest possible valve triggering times are feasible; because of its substantially shorter reaction times, a piezoelectric actuator is superior to electromagnetic actuators. Clocking of the actuator positioning signal, when a piezoelectric actuator is used, is converted virtually directly into a clocked motion of the triggered control valve or control valves. With electromagnets, it is not feasible to convert the trigger signal directly into adjusting motions of the control valves acted upon, and so the clock signal would be wrong, and inappropriate courses of motion would ensue.
With the method proposed according to the invention, because of the short response times of the actuating devices used, it is possible to perform a multiple, clocked triggering of the nozzle control valve, so that upon starting of an internal combustion engine, an adequate quantity of fuel can be injected. Unwanted closure of the nozzle needle precisely during the starting phase, as can happen in the embodiments sketched above in the background section, is precluded in the method proposed according to the invention, because of the fast response times. By clocked opening and closing of the nozzle control valve during the injection event, the quantity of leakage at medium rpm is reduced. The result is an increase in the peak pressure or in the injection quantity, for the same total duration of triggering the control valves. With the method proposed according to the invention, thanks to the maximally short valve control times achieved by the piezoelectric actuator, the efficiency of the nozzle control valve can be increased. If at medium rpm of an internal combustion engine used in a utility vehicle, the nozzle control valve is opened and closed in clocked fashion during the injection event, then the resultant leakage can be reduced, and a better degree of filling of the particular combustion chamber of an internal combustion engine can be attained. The peak pressure and the injection quantity both increase at medium rpm of the engine, so that the thermodynamic variables that affect the efficiency have a positive development.
At the rated rpm, for which an internal combustion engine is as a rule designed, the same positive effect of a small leakage quantity can be attained if the nozzle control valve is briefly opened and closed multiple times. At rated rpm, with a brief opening and closure of the nozzle control valve and with the fill diversion valve kept closed, improved filling of the combustion chambers of the engine is attainable, which increases efficiency by improving fuel utilization.